1. Field of the Invention:
The invention relates to the techniques of cooling glass sheets after they have been heated, in particular for bending them.
2. Background of the Related Art:
When it is desired to give a glass sheet a nonplanar shape, for example during the manufacture of an automobile glazing, the planar sheet is brought to a temperature higher than the glass transformation temperature, deformation is caused either by stressing the malleable glass to assume a rigid shape, or by positioning it horizontally and allowing it to become deformed under its own weight. Once the desired shape is obtained, a controlled cooling of the glass is carried out. The most recently employed cooling processes are tempering and annealing.
The first, which general comprises energetically blowing on the glass, provides it, once cooled, with a prestressed state which increases its resistance to bending and thermal shock. This process is used typically for manufacturing of glazings which are to equip the sides or the rear of automobiles.
In annealing, on the other hand, the relaxation of stresses, is permitted by cooling the glass in a very gradual manner. In this case, an annealed glass is obtained whose stress level is slight in the direction of the thickness of the glass, which optionally allows the finished product to be cut and which, in the case of an automobile glazing, avoids explosive breakage in case by an impact of fine gravel. This technique is therefore particularly adapted to the manufacturing of windshields.
Also known is a technique where two sheets of superimposed glass which are simultaneously heated, bent and cooled are then assembled in pairs with a plastic sheet inserted therebetween.
But an automobile windshield is subjected to handling conditions before its final assembly on the automobile production line, or in conditions of use after putting a vehicle on the road, which require a suitable mechanical performance. Annealed glass is fragile and it is advisable to limit the risks of breakage either at the time of assembly handling or when in the car, for example, breakage by thermal shock when hot air is blown on the inner surface of a cold windshield by the defogger or defroster.
A well-known process consists of moderately blowing on the periphery of the glass sheets at the beginning of the cooling process to create in this peripheral zone a slight compression prestressing which limits the risks of later breakage. Thus, for example, in the bending furnace for laminated glazings described in French patent application No. FR 87-16 083, there is positioned downstream from the cells where the bending of the glass is carried out by gravity, a blowing cell where, during the holding time of the glass, a cooling gas is blown on the peripheral zone of the sheet.
To carry out this blowing, blowing hoods are usually used which have a truncated pyramid shape. The air is guided by the walls of the hood and then escapes in a free passage between the edge of the hood and the glass. In the center, the air circulation is much slower: the heat exchange is therefore increased at the periphery. To be fully satisfactory, this system requires that the dimensions of the glass sheet and the hood be adapted to each other. If it is desired to treat sheets of different dimensions with the same hood, satisfactory performance becomes difficult. In particular, for small-series cars or when the demand is very diversified as in the market for replacement products, it may be desired to alternate the production of different models. It would be necessary in this case to change the blowing hood when changing the model. This is not possible since the adaptation possibilities of a system having a given hood are slight. Only the flow of air can be varied, either by changing the pressure upstream from the hood or by varying the distance of the hood from the glass. But, in doing this the treated surface is also changed as is the distribution of air pressure on its surface. It is practically impossible under these conditions, and on models of different sizes and thicknesses, to master both the width of the prestressed zone on the entire periphery of the glass sheet and the value of its prestressing.
Novel system designs may be considered for carrying out the tempering of glass of varied dimensions. Thus European patent application No. EP 0 246 123 proposes the use of a box for adjustable blowing, where slide valves allow certain zones to be sealed. Such a device could be used for closing more of the central part of the box as the windshield becomes larger. But the mechanical controls of these systems are delicate and their operation in the cells, which are hot and difficult to access, would pose problems which the present invention makes it possible to avoid.